1. Field of the Invention
This invention relates to the field of control systems for inductive electrical loads, including motors, and in particular, to controlling the amount of current flowing through an inductive load.
2. Background Art
Electrical circuits often include inductive components. In some circuits, the amount of current required by the inductive components varies over time. To optimize performance of an electrical system and components such as power supplies and switching devices, it may be desirable to limit the amount of current which may flow through inductive components.
Motors are one type of inductive component for which it is desirable to control operating current. In the prior art, two methods have been used to control current in motors. One prior art method is known as linear control. Linear current control allows a steady current to flow at a controlled level. A second method of the prior art is known as pulse width modulation (PWM). Pulse width modulation causes current to be applied in pulses of a fixed level. Pulse width modulation allows current to be controlled by varying the width of the pulses. By switching current fully on or fully off, pulse width modulation achieves great efficiency by avoiding losses associated with linear current control. However, by generating pulses, pulse width modulation can introduce electrical and acoustic noise into the systems in which it is employed. Such noise can interfere with other components of the system and can annoy the users of the system.
Certain prior art motor controllers have allowed operation of motors in either the linear or PWM modes. However, these motor controllers do not allow automatic selection of linear or PWM operation. The operating mode of the controller (linear or PWM) must be selected by circuits external to the controller circuit and thus, the controller may be in the linear mode when it should be in the PWM mode, resulting in decreased efficiency. Also, the motor may be in the PWM mode when it should be left in the linear mode, increasing the presence of noise in the system.
One prior art motor controller is the Micro Linear ML4411. The ML4411 allows linear and PWM operation, but uses variable frequency PWM, which requires a one-shot timing circuit. Also, the ML4411 requires upper and lower drivers of opposite semiconductor-type polarities.